Molecular beam epitaxial growth of high-quality InSb on InP and GaAs substrates

Epitaxial layers of InSb were grown on InP and GaAs substrates by molecular beam epitaxy. The dependence of the epilayer quality on flux ratio, J sub Sb4/J sub In, was studied. Deviation from an optimum value of J sub Sb4/J sub In (approx. 2) during growth led to deterioration in the surface morphology and the electrical and crystalline qualities of the films. Room temperature electron mobilities as high as 70,000 and 53,000 sq cm /V-s were measured in InSb layers grown on InP and GaAs substrates, respectively. Unlike the previous results, the conductivity in these films is n-type even at T = 13 K, and no degradation of the electron mobility due to the high density of dislocations was observed. The measured electron mobilities (and carrier concentrations) at 77 K in InSb layers grown on InP and GaAs substrates are 110,000 sq cm/V-s (3 x 10(15) cm(-3)) and 55,000 sq cm/V-s (4.95 x 10(15) cm(-3)), respectively, suggesting their application to electronic devices at cryogenic temperatures

First-principles study on atomic configuration of electron-beam irradiated C60 film

Density functional calculations of the atomic configuration of electron-beam irradiated C-60 thin films were implemented. By examining the electronic structure and electron-transport properties of C-60 clusters, we found that a rhombohedral C-60 polymer with sp(3)-bonded dumbbell-shaped connections at the molecule junction is a semiconductor with a narrow band gap. In addition, the polymer changes to exhibit metallic behavior by forming sp(2)-bonded peanut-shaped connections. Conductance below the Fermi level increases and the peak of the conductance spectrum arising from the t(u1) states of the C-60 molecule becomes obscure after the connections are rearranged. The present rhombohedral polymer, including the [2 + 2] four-membered rings and peanut-shaped connections, is a candidate for representing the structure of the metallic C-60 polymer at the initial stage of electron-beam irradiation

Statistical Analysis of Surface Reconstruction Domains on InAs Wetting Layer Preceding Quantum Dot Formation

Surface of an InAs wetting layer on GaAs(001) preceding InAs quantum dot (QD) formation was observed at 300°C with in situ scanning tunneling microscopy (STM). Domains of (1 × 3)/(2 × 3) and (2 × 4) surface reconstructions were located in the STM image. The density of each surface reconstruction domain was comparable to that of subsequently nucleated QD precursors. The distribution of the domains was statistically investigated in terms of spatial point patterns. It was found that the domains were distributed in an ordered pattern rather than a random pattern. It implied the possibility that QD nucleation sites are related to the surface reconstruction domains

Josephson junction in cobalt-doped BaFe2As2 epitaxial thin films on (La, Sr)(Al, Ta)O3 bicrystal substrates

Josephson junctions were fabricated in epitaxial films of cobalt-doped BaFe2As2 on [001]-tilt (La,Sr)(Al,Ta)O3 bicrystal substrates. 10m-wide microbridges spanning a 30-degrees-tilted bicrystal grain boundary (BGB bridge) exhibited resistively-shunted-junction (RSJ)-like current-voltage characteristics up to 17 K, and the critical current was suppressed remarkably by a magnetic field. Microbridges without a BGB did not show the RSJ-like behavior, and their critical current densities were 20 times larger than those of BGB bridges, confirming BGB bridges display a Josephson effect originating from weakly-linked BGB

The spin-incoherent Luttinger liquid

In contrast to the well known Fermi liquid theory of three dimensions, interacting one-dimensional and quasi one-dimensional systems of fermions are described at low energy by an effective theory known as Luttinger liquid theory. This theory is expressed in terms of collective many-body excitations that show exotic behavior such as spin-charge separation. Luttinger liquid theory is commonly applied on the premise that "low energy" describes both the spin and charge sectors. However, when the interactions in the system are very strong, as they typically are at low particle densities, the ratio of spin to charge energy may become exponentially small. It is then possible at very low temperatures for the energy to be low compared to the characteristic charge energy, but still high compared to the characteristic spin energy. This energy window of near ground-state charge degrees of freedom, but highly thermally excited spin degrees of freedom is called a spin-incoherent Luttinger liquid. The spin-incoherent Luttinger liquid exhibits a higher degree universality than the Luttinger liquid and its properties are qualitatively distinct. In this colloquium I detail some of the recent theoretical developments in the field and describe experimental indications of such a regime in gated semiconductor quantum wires.Comment: 21 pages, 18 figures. Updated references, corrected typo in Eq.(20) in journal versio

Temperature-Dependent Site Control of InAs/GaAs (001) Quantum Dots Using a Scanning Tunneling Microscopy Tip During Growth

Site-controlled InAs nano dots were successfully fabricated by a STMBE system (in situ scanning tunneling microscopy during molecular beam epitaxy growth) at substrate temperatures from 50 to 430°C. After 1.5 ML of the InAs wetting layer (WL) growth by ordinal Stranski–Krastanov dot fabrication procedures, we applied voltage at particular sites on the InAs WL, creating the site where In atoms, which were migrating on the WL, favored to congregate. At 240°C, InAs nano dots (width: 20–40 nm, height: 1.5–2.0 nm) were fabricated. At 430°C, InAs nano dots (width: 16–20 nm, height: 0.75–1.5 nm) were also fabricated. However, these dots were remained at least 40 s and collapsed less than 1000 s. Then, we fabricated InAs nano dots (width: 24–150 nm, height: 2.8–28 nm) at 300°C under In and As4 irradiations. These were not collapsed and considered to high crystalline dots

Electron-Transport Properties of Na Nanowires under Applied Bias Voltages

We present first-principles calculations on electron transport through Na nanowires at finite bias voltages. The nanowire exhibits a nonlinear current-voltage characteristic and negative differential conductance. The latter is explained by the drastic suppression of the transmission peaks which is attributed to the electron transportability of the negatively biased plinth attached to the end of the nanowire. In addition, the finding that a voltage drop preferentially occurs on the negatively biased side of the nanowire is discussed in relation to the electronic structure and conduction.Comment: 4 pages, 6 figure